1. Field of the Invention
This invention relates to synchronous motor systems, and particularly stepping motor drives which use chopping techniques to regulate currents in the motor windings. As used herein, the term synchronous motor refers to any motor whose advance depends upon the frequency of the power input.
A typical motor driver energizes the separate windings of a stepping motor with separate power switching circuits, such as bridge circuits, that direct the currents back and forth through the respective windings. A current-regulating chopper responds when the current in each winding exceeds a set value and turns off selected switches in the bridges. Freewheeling diodes in the bridge then direct the currents, formed by the collapsing field about each winding, around a path until the winding current drops below a given value. The chopper then allows the switches in the bridge to resume directing current through the winding until the current again rises above the set value or until the phase of the winding's current energization has been completed.
Two types of chopper controls exist. In one, the current formed by the field collapsing around the winding is circulated through a switch of the bridge and ground, and then a freewheeling diode. In another, non-circulating drive, a monostable multivibrator establishes the chopper time response to the winding current and turns off all the bridge's switches so that the current formed by the resulting collapsing field about the motor winding passes through the freewheeling diodes back to the source energizing the bridge against, the potential of the source.
A major disadvantage of circulating drives reside in the fact the chopping frequency varies with motor inductance. The range of frequency is typically between 2,000 and 15,000 Hz which causes unwanted audible noise. Drives with circulating choppers provide no control over peak currents when the motor is braking. In this situation the back EMF of the motor voltage is greater than the driver voltage and the resultant motor current may increase uncontrollably above the previously set value of the regulation loop.
The noncirculating choppers suffer from other disadvantages. Because the ripple current produced by the chopping is proportional to the supply voltage, the percent of ripple current becomes excessively large when operating with power supplies greater than 50 volts. The current regulator circuits for each of the two windings in the motor are nonsynchronous. Hence, the peak current flowing into and out of the power supply can be as large as two times the peak winding current. This causes excessive heating of the capacitors in the source, high electrical noise in the power supply and interconnecting cables, and a time varying ripple voltage on the power supply which causes motor current modulation. The large ripple currents make eddy current losses a significant factor and limit the output power delivered to a load. Rapid switching signals result in electrical noise that can couple to external electronic equipment.
An object of the invention is to improve synchronous motor systems.
Another object of the invention is to avoid the aforementioned disadvantages, either in whole or in part.